RadarRadar is an object-detection system that uses radio waves to determine
the range, angle, or velocity of objects. It can be used to detect
aircraft, ships, spacecraft, guided missiles, motor vehicles, weather
formations, and terrain. A radar system consists of a transmitter
producing electromagnetic waves in the radio or microwaves domain, a
transmitting antenna, a receiving antenna (often the same antenna is
used for transmitting and receiving) and a receiver and processor to
determine properties of the object(s).
RadioRadio waves (pulsed or
continuous) from the transmitter reflect off the object and return to
the receiver, giving information about the object's location and
speed.
RadarRadar was developed secretly for military use by several nations in
the period before and during World War II. A key development was the
cavity magnetron in the UK, which allowed the creation of relatively
small systems with sub-meter resolution. The term RADAR was coined in
1940 by the
United States NavyUnited States Navy as an acronym for RAdio
Detection And
Ranging[1][2] or RAdio Direction And Ranging.[3][4] The term radar has
since entered English and other languages as a common noun, losing all
capitalization.
The modern uses of radar are highly diverse, including air and
terrestrial traffic control, radar astronomy, air-defence systems,
antimissile systems, marine radars to locate landmarks and other
ships, aircraft anticollision systems, ocean surveillance systems,
outer space surveillance and rendezvous systems, meteorological
precipitation monitoring, altimetry and flight control systems, guided
missile target locating systems, ground-penetrating radar for
geological observations, and range-controlled radar for public health
surveillance.[5] High tech radar systems are associated with digital
signal processing, machine learning and are capable of extracting
useful information from very high noise levels.
Other systems similar to radar make use of other parts of the
electromagnetic spectrum. One example is "lidar", which uses
predominantly infrared light from lasers rather than radio waves.

Contents

1 History

1.1 First experiments
1.2 Just before World War II
1.3 During World War II

History[edit]
Main article: History of radar
First experiments[edit]
As early as 1886, German physicist
Heinrich HertzHeinrich Hertz showed that radio
waves could be reflected from solid objects. In 1895, Alexander Popov,
a physics instructor at the
Imperial Russian NavyImperial Russian Navy school in Kronstadt,
developed an apparatus using a coherer tube for detecting distant
lightning strikes. The next year, he added a spark-gap transmitter. In
1897, while testing this equipment for communicating between two ships
in the Baltic Sea, he took note of an interference beat caused by the
passage of a third vessel. In his report, Popov wrote that this
phenomenon might be used for detecting objects, but he did nothing
more with this observation.[6]
The German inventor
Christian Hülsmeyer was the first to use radio
waves to detect "the presence of distant metallic objects". In 1904,
he demonstrated the feasibility of detecting a ship in dense fog, but
not its distance from the transmitter.[7] He obtained a patent[8] for
his detection device in April 1904 and later a patent[9] for a related
amendment for estimating the distance to the ship. He also got a
British patent on September 23, 1904[10] for a full radar system, that
he called a telemobiloscope. It operated on a 50 cm wavelength
and the pulsed radar signal was created via a spark-gap. His system
already used the classic antenna setup of horn antenna with parabolic
reflector and was presented to German military officials in practical
tests in
CologneCologne and
RotterdamRotterdam harbour but was rejected.[11]
In 1915,
Robert Watson-WattRobert Watson-Watt used radio technology to provide advance
warning to airmen[12] and during the 1920s went on to lead the U.K.
research establishment to make many advances using radio techniques,
including the probing of the ionosphere and the detection of lightning
at long distances. Through his lightning experiments, Watson-Watt
became an expert on the use of radio direction finding before turning
his inquiry to shortwave transmission. Requiring a suitable receiver
for such studies, he told the "new boy"
Arnold Frederic WilkinsArnold Frederic Wilkins to
conduct an extensive review of available shortwave units. Wilkins
would select a
General Post OfficeGeneral Post Office model after noting its manual's
description of a "fading" effect (the common term for interference at
the time) when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver
on opposite sides of the Potomac River, U.S. Navy researchers A. Hoyt
Taylor and
Leo C. Young discovered that ships passing through the beam
path caused the received signal to fade in and out. Taylor submitted a
report, suggesting that this phenomenon might be used to detect the
presence of ships in low visibility, but the Navy did not immediately
continue the work. Eight years later,
Lawrence A. Hyland at the Naval
Research Laboratory (NRL) observed similar fading effects from passing
aircraft; this revelation led to a patent application[13] as well as a
proposal for further intensive research on radio-echo signals from
moving targets to take place at NRL, where Taylor and Young were based
at the time.[14]
Just before World War II[edit]

Before the Second World War, researchers in the United Kingdom,
France, Germany, Italy, Japan, the Netherlands, the Soviet Union, and
the United States, independently and in great secrecy, developed
technologies that led to the modern version of radar. Australia,
Canada, New Zealand, and
South AfricaSouth Africa followed prewar Great Britain's
radar development, and Hungary generated its radar technology during
the war.[15]
In France in 1934, following systematic studies on the Split Anode
Magnetron, the research branch of the Compagnie Générale de
Télégraphie Sans Fil (CSF) headed by Maurice Ponte with Henri
Gutton, Sylvain Berline and M. Hugon, began developing an
obstacle-locating radio apparatus, aspects of which were installed on
the ocean liner Normandie in 1935.[16][17]
During the same period, Soviet military engineer P. K. Oshchepkov, in
collaboration with Leningrad Electrophysical Institute, produced an
experimental apparatus, RAPID, capable of detecting an aircraft within
3 km of a receiver.[18] The Soviets produced their first mass
production radars RUS-1 and RUS-2 Redut in 1939 but further
development was slowed following the arrest of Oshchepkov and his
subsequent gulag sentence. In total, only 607 Redut stations were
produced during the war. The first Russian airborne radar, Gneiss-2,
entered into service in June 1943 on Pe-2 fighters. More than 230
Gneiss-2 stations were produced by the end of 1944.[19] The French and
Soviet systems, however, featured continuous-wave operation that did
not provide the full performance ultimately synonymous with modern
radar systems.
Full radar evolved as a pulsed system, and the first such elementary
apparatus was demonstrated in December 1934 by the American Robert M.
Page, working at the Naval Research Laboratory.[20] The following
year, the
United StatesUnited States Army successfully tested a primitive
surface-to-surface radar to aim coastal battery searchlights at
night.[21] This design was followed by a pulsed system demonstrated in
May 1935 by
Rudolf Kühnhold and the firm GEMA in Germany and then
another in June 1935 by an
Air MinistryAir Ministry team led by Robert A.
Watson-Watt in Great Britain.

In 1935, Watson-Watt was asked to judge recent reports of a German
radio-based death ray and turned the request over to Wilkins. Wilkins
returned a set of calculations demonstrating the system was basically
impossible. When Watson-Watt then asked what such a system might do,
Wilkins recalled the earlier report about aircraft causing radio
interference. This revelation led to the
Daventry ExperimentDaventry Experiment of 26
February 1935, using a powerful
BBCBBC shortwave transmitter as the
source and their GPO receiver setup in a field while a bomber flew
around the site. When the plane was clearly detected, Hugh Dowding,
the
Air Member for Supply and Research was very impressed with their
system's potential and funds were immediately provided for further
operational development.[22] Watson-Watt's team patented the device in
GB593017.[23][24][25]
Development of radar greatly expanded on 1 September 1936 when
Watson-Watt became Superintendent of a new establishment under the
British Air Ministry, Bawdsey Research Station located in Bawdsey
Manor, near Felixstowe, Suffolk. Work there resulted in the design and
installation of aircraft detection and tracking stations called "Chain
Home" along the East and South coasts of England in time for the
outbreak of
World War IIWorld War II in 1939. This system provided the vital
advance information that helped the Royal Air Force win the Battle of
Britain; without it, significant numbers of fighter aircraft would
always need to be in the air to respond quickly enough if enemy
aircraft detection relied solely on the observations of ground-based
individuals. Also vital was the "Dowding system" of reporting and
coordination to make best use of the radar information during tests of
early deployment of radar in 1936 and 1937.
Given all required funding and development support, the team produced
working radar systems in 1935 and began deployment. By 1936, the first
five
Chain HomeChain Home (CH) systems were operational and by 1940 stretched
across the entire UK including Northern Ireland. Even by standards of
the era, CH was crude; instead of broadcasting and receiving from an
aimed antenna, CH broadcast a signal floodlighting the entire area in
front of it, and then used one of Watson-Watt's own radio direction
finders to determine the direction of the returned echoes. This fact
meant CH transmitters had to be much more powerful and have better
antennas than competing systems but allowed its rapid introduction
using existing technologies.
During World War II[edit]
Main article:
RadarRadar in World War II
A key development was the cavity magnetron in the UK, which allowed
the creation of relatively small systems with sub-meter resolution.
Britain shared the technology with the U.S. during the 1940 Tizard
Mission.[26][27]
In April 1940,
Popular SciencePopular Science showed an example of a radar unit using
the Watson-Watt patent in an article on air defence.[28] Also, in late
1941 Popular Mechanics had an article in which a U.S. scientist
speculated about the British early warning system on the English east
coast and came close to what it was and how it worked.[29] Watson-Watt
was sent to the U.S. in 1941 to advise on air defense after Japan’s
attack on Pearl Harbor.[30]
Alfred Lee LoomisAlfred Lee Loomis organized the Radiation
Laboratory at Cambridge, Massachusetts which developed the technology
in the years 1941–45. Later, in 1943, Page greatly improved radar
with the monopulse technique that was used for many years in most
radar applications.[31]
The war precipitated research to find better resolution, more
portability, and more features for radar, including complementary
navigation systems like Oboe used by the RAF's Pathfinder.
Applications[edit]

The information provided by radar includes the bearing and range (and
therefore position) of the object from the radar scanner. It is thus
used in many different fields where the need for such positioning is
crucial. The first use of radar was for military purposes: to locate
air, ground and sea targets. This evolved in the civilian field into
applications for aircraft, ships, and roads.
In aviation, aircraft can be equipped with radar devices that warn of
aircraft or other obstacles in or approaching their path, display
weather information, and give accurate altitude readings. The first
commercial device fitted to aircraft was a 1938 Bell Lab unit on some
United Air LinesUnited Air Lines aircraft.[29]
AircraftAircraft can land in fog at airports
equipped with radar-assisted ground-controlled approach systems in
which the plane's position is observed on radar screens by operators
who radio landing instructions to the pilot, maintaining the aircraft
on a defined approach path to the runway.
MilitaryMilitary fighter aircraft
are usually fitted with air-to-air targeting radars, to detect and
target enemy aircraft. In addition, larger specialized military
aircraft carry powerful airborne radars to observe air traffic over a
wide region and direct fighter aircraft towards targets.
Marine radars are used to measure the bearing and distance of ships to
prevent collision with other ships, to navigate, and to fix their
position at sea when within range of shore or other fixed references
such as islands, buoys, and lightships. In port or in harbour, vessel
traffic service radar systems are used to monitor and regulate ship
movements in busy waters.
Meteorologists use radar to monitor precipitation and wind. It has
become the primary tool for short-term weather forecasting and
watching for severe weather such as thunderstorms, tornadoes, winter
storms, precipitation types, etc. Geologists use specialized
ground-penetrating radars to map the composition of Earth's crust.
Police forces use radar guns to monitor vehicle speeds on the roads.
Smaller radar systems are used to detect human movement. Examples are
breathing pattern detection for sleep monitoring[32] and hand and
finger gesture detection for computer interaction.[33] Automatic door
opening, light activation and intruder sensing are also common.
Principles[edit]
RadarRadar signal[edit]
Further information:
RadarRadar signal characteristics
A radar system has a transmitter that emits radio waves called radar
signals in predetermined directions. When these come into contact with
an object they are usually reflected or scattered in many directions.
But some of them absorb and penetrate into the target to some degree.
RadarRadar signals are reflected especially well by materials of
considerable electrical conductivity—especially by most metals, by
seawater and by wet ground. Some of these make the use of radar
altimeters possible. The radar signals that are reflected back towards
the transmitter are the desirable ones that make radar work. If the
object is moving either toward or away from the transmitter, there is
a slight equivalent change in the frequency of the radio waves, caused
by the Doppler effect.
RadarRadar receivers are usually, but not always, in the same location as
the transmitter. Although the reflected radar signals captured by the
receiving antenna are usually very weak, they can be strengthened by
electronic amplifiers. More sophisticated methods of signal processing
are also used in order to recover useful radar signals.
The weak absorption of radio waves by the medium through which it
passes is what enables radar sets to detect objects at relatively long
ranges—ranges at which other electromagnetic wavelengths, such as
visible light, infrared light, and ultraviolet light, are too strongly
attenuated. Such weather phenomena as fog, clouds, rain, falling snow,
and sleet that block visible light are usually transparent to radio
waves. Certain radio frequencies that are absorbed or scattered by
water vapour, raindrops, or atmospheric gases (especially oxygen) are
avoided in designing radars, except when their detection is intended.
Illumination[edit]
RadarRadar relies on its own transmissions rather than light from the Sun
or the Moon, or from electromagnetic waves emitted by the objects
themselves, such as infrared wavelengths (heat). This process of
directing artificial radio waves towards objects is called
illumination, although radio waves are invisible to the human eye or
optical cameras.
Reflection[edit]
Main article: Reflection (physics)

Brightness can indicate reflectivity as in this 1960 weather radar
image (of Hurricane Abby). The radar's frequency, pulse form,
polarization, signal processing, and antenna determine what it can
observe.

If electromagnetic waves travelling through one material meet another
material, having a different dielectric constant or diamagnetic
constant from the first, the waves will reflect or scatter from the
boundary between the materials. This means that a solid object in air
or in a vacuum, or a significant change in atomic density between the
object and what is surrounding it, will usually scatter radar (radio)
waves from its surface. This is particularly true for electrically
conductive materials such as metal and carbon fibre, making radar
well-suited to the detection of aircraft and ships.
RadarRadar absorbing
material, containing resistive and sometimes magnetic substances, is
used on military vehicles to reduce radar reflection. This is the
radio equivalent of painting something a dark colour so that it cannot
be seen by the eye at night.
RadarRadar waves scatter in a variety of ways depending on the size
(wavelength) of the radio wave and the shape of the target. If the
wavelength is much shorter than the target's size, the wave will
bounce off in a way similar to the way light is reflected by a mirror.
If the wavelength is much longer than the size of the target, the
target may not be visible because of poor reflection. Low-frequency
radar technology is dependent on resonances for detection, but not
identification, of targets. This is described by Rayleigh scattering,
an effect that creates Earth's blue sky and red sunsets. When the two
length scales are comparable, there may be resonances. Early radars
used very long wavelengths that were larger than the targets and thus
received a vague signal, whereas many modern systems use shorter
wavelengths (a few centimetres or less) that can image objects as
small as a loaf of bread.
Short radio waves reflect from curves and corners in a way similar to
glint from a rounded piece of glass. The most reflective targets for
short wavelengths have 90° angles between the reflective surfaces. A
corner reflector consists of three flat surfaces meeting like the
inside corner of a box. The structure will reflect waves entering its
opening directly back to the source. They are commonly used as radar
reflectors to make otherwise difficult-to-detect objects easier to
detect. Corner reflectors on boats, for example, make them more
detectable to avoid collision or during a rescue. For similar reasons,
objects intended to avoid detection will not have inside corners or
surfaces and edges perpendicular to likely detection directions, which
leads to "odd" looking stealth aircraft. These precautions do not
completely eliminate reflection because of diffraction, especially at
longer wavelengths. Half wavelength long wires or strips of conducting
material, such as chaff, are very reflective but do not direct the
scattered energy back toward the source. The extent to which an object
reflects or scatters radio waves is called its radar cross section.
RadarRadar equation[edit]
The power Pr returning to the receiving antenna is given by the
equation:

Pt = transmitter power
Gt = gain of the transmitting antenna
Ar = effective aperture (area) of the receiving antenna; this can also
be expressed as

G

r

λ

2

4
π

displaystyle G_ r lambda ^ 2 over 4pi

, where

λ

displaystyle lambda

= transmitted wavelength
Gr = gain of receiving antenna[34]

σ = radar cross section, or scattering coefficient, of the target
F = pattern propagation factor
Rt = distance from the transmitter to the target
Rr = distance from the target to the receiver.

In the common case where the transmitter and the receiver are at the
same location, Rt = Rr and the term Rt² Rr² can be replaced by R4,
where R is the range. This yields:

P

r

=

P

t

G

t

A

r

σ

F

4

(
4
π
)

2

R

4

.

displaystyle P_ r = P_ t G_ t A_ r sigma F^ 4 over (4pi ) ^
2 R^ 4 .

This shows that the received power declines as the fourth power of the
range, which means that the received power from distant targets is
relatively very small.
Additional filtering and pulse integration modifies the radar equation
slightly for pulse-
Doppler radarDoppler radar performance, which can be used to
increase detection range and reduce transmit power.
The equation above with F = 1 is a simplification for transmission in
a vacuum without interference. The propagation factor accounts for the
effects of multipath and shadowing and depends on the details of the
environment. In a real-world situation, pathloss effects should also
be considered.
Doppler effect[edit]
Main articles:
Doppler radarDoppler radar and
Pulse-DopplerPulse-Doppler radar
FrequencyFrequency shift is caused by motion that changes the number of
wavelengths between the reflector and the radar. This can degrade or
enhance radar performance depending upon how it affects the detection
process. As an example,
Moving Target IndicationMoving Target Indication can interact with
Doppler to produce signal cancellation at certain radial velocities,
which degrades performance.
Sea-based radar systems, semi-active radar homing, active radar
homing, weather radar, military aircraft, and radar astronomy rely on
the
Doppler effectDoppler effect to enhance performance. This produces information
about target velocity during the detection process. This also allows
small objects to be detected in an environment containing much larger
nearby slow moving objects.
Doppler shift depends upon whether the radar configuration is active
or passive. Active radar transmits a signal that is reflected back to
the receiver. Passive radar depends upon the object sending a signal
to the receiver.
The Doppler frequency shift for active radar is as follows, where

F

D

displaystyle F_ D

is Doppler frequency,

F

T

displaystyle F_ T

is transmit frequency,

V

R

displaystyle V_ R

is radial velocity, and

C

displaystyle C

is the speed of light:[35]

F

D

=
2
×

F

T

×

(

V

R

C

)

displaystyle F_ D =2times F_ T times left( frac V_ R C right)

.

Passive radar is applicable to electronic countermeasures and radio
astronomy as follows:

F

D

=

F

T

×

(

V

R

C

)

displaystyle F_ D =F_ T times left( frac V_ R C right)

.

Only the radial component of the velocity is relevant. When the
reflector is moving at right angle to the radar beam, it has no
relative velocity. Vehicles and weather moving parallel to the radar
beam produce the maximum Doppler frequency shift.
When the transmit frequency (

. As a result, the Doppler measurement is only non-ambiguous if the
Doppler frequency shift is less than half of

F

R

displaystyle F_ R

, called the Nyquist frequency, since the returned frequency otherwise
cannot be distinguished from shifting of a harmonic frequency above or
below, thus requiring:

F

D

<

F

R

2

displaystyle F_ D < frac F_ R 2

Or when substituting with

F

D

displaystyle F_ D

:

V

R

<

F

R

×

C

F

T

4

displaystyle V_ R < frac F_ R times frac C F_ T 4

As an example, a Doppler weather radar with a pulse rate of 2 kHz
and transmit frequency of 1 GHz can reliably measure weather
speed up to at most 150 m/s (340 mph), thus cannot reliably
determine radial velocity of aircraft moving 1,000 m/s
(2,200 mph).
Polarization[edit]
Further information: Polarization (waves)
In all electromagnetic radiation, the electric field is perpendicular
to the direction of propagation, and the electric field direction is
the polarization of the wave. For a transmitted radar signal, the
polarization can be controlled to yield different effects. Radars use
horizontal, vertical, linear, and circular polarization to detect
different types of reflections. For example, circular polarization is
used to minimize the interference caused by rain. Linear polarization
returns usually indicate metal surfaces. Random polarization returns
usually indicate a fractal surface, such as rocks or soil, and are
used by navigation radars.
Limiting factors[edit]
Beam path and range[edit]
See also:
Beam formingBeam forming and Over-the-horizon radar

Echo heights above ground

The radar beam would follow a linear path in vacuum, but it really
follows a somewhat curved path in the atmosphere because of the
variation of the refractive index of air, that is called the radar
horizon. Even when the beam is emitted parallel to the ground, it will
rise above it as the Earth curvature sinks below the horizon.
Furthermore, the signal is attenuated by the medium it crosses, and
the beam disperses.
The maximum range of a conventional radar can be limited by a number
of factors:

Line of sight, which depends on height above ground. This means
without a direct line of sight the path of the beam is blocked.
The maximum non-ambiguous range, which is determined by the pulse
repetition frequency. The maximum non-ambiguous range is the distance
the pulse could travel and return before the next pulse is emitted.
RadarRadar sensitivity and power of the return signal as computed in the
radar equation. This includes factors such as environmental conditions
and the size (or radar cross section) of the target.

Noise[edit]
Main articles:
Noise (electronics)Noise (electronics) and Noise (radio)
Signal noise is an internal source of random variations in the signal,
which is generated by all electronic components.
Reflected signals decline rapidly as distance increases, so noise
introduces a radar range limitation. The noise floor and signal to
noise ratio are two different measures of performance that affect
range performance. Reflectors that are too far away produce too little
signal to exceed the noise floor and cannot be detected. Detection
requires a signal that exceeds the noise floor by at least the signal
to noise ratio.
Noise typically appears as random variations superimposed on the
desired echo signal received in the radar receiver. The lower the
power of the desired signal, the more difficult it is to discern it
from the noise.
Noise figure is a measure of the noise produced by a
receiver compared to an ideal receiver, and this needs to be
minimized.
Shot noiseShot noise is produced by electrons in transit across a discontinuity,
which occurs in all detectors.
Shot noiseShot noise is the dominant source in
most receivers. There will also be flicker noise caused by electron
transit through amplification devices, which is reduced using
heterodyne amplification. Another reason for heterodyne processing is
that for fixed fractional bandwidth, the instantaneous bandwidth
increases linearly in frequency. This allows improved range
resolution. The one notable exception to heterodyne (downconversion)
radar systems is ultra-wideband radar. Here a single cycle, or
transient wave, is used similar to UWB communications, see List of UWB
channels.
Noise is also generated by external sources, most importantly the
natural thermal radiation of the background surrounding the target of
interest. In modern radar systems, the internal noise is typically
about equal to or lower than the external noise. An exception is if
the radar is aimed upwards at clear sky, where the scene is so "cold"
that it generates very little thermal noise. The thermal noise is
given by kB T B, where T is temperature, B is bandwidth (post matched
filter) and kB is Boltzmann's constant. There is an appealing
intuitive interpretation of this relationship in a radar. Matched
filtering allows the entire energy received from a target to be
compressed into a single bin (be it a range, Doppler, elevation, or
azimuth bin). On the surface it would appear that then within a fixed
interval of time one could obtain perfect, error free, detection. To
do this one simply compresses all energy into an infinitesimal time
slice. What limits this approach in the real world is that, while time
is arbitrarily divisible, current is not. The quantum of electrical
energy is an electron, and so the best one can do is match filter all
energy into a single electron. Since the electron is moving at a
certain temperature (Plank spectrum) this noise source cannot be
further eroded. We see then that radar, like all macro-scale entities,
is profoundly impacted by quantum theory.
Noise is random and target signals are not.
Signal processingSignal processing can take
advantage of this phenomenon to reduce the noise floor using two
strategies. The kind of signal integration used with moving target
indication can improve noise up to

2

displaystyle sqrt 2

for each stage. The signal can also be split among multiple filters
for pulse-Doppler signal processing, which reduces the noise floor by
the number of filters. These improvements depend upon coherence.
Interference[edit]
Main article: Interference (wave propagation)
RadarRadar systems must overcome unwanted signals in order to focus on the
targets of interest. These unwanted signals may originate from
internal and external sources, both passive and active. The ability of
the radar system to overcome these unwanted signals defines its
signal-to-noise ratio (SNR). SNR is defined as the ratio of the signal
power to the noise power within the desired signal; it compares the
level of a desired target signal to the level of background noise
(atmospheric noise and noise generated within the receiver). The
higher a system's SNR the better it is at discriminating actual
targets from noise signals.
Clutter[edit]
Main article: Clutter (radar)
Clutter refers to radio frequency (RF) echoes returned from targets
which are uninteresting to the radar operators. Such targets include
natural objects such as ground, sea, and when not being tasked for
meteorological purposes, precipitation (such as rain, snow or hail),
sand storms, animals (especially birds), atmospheric turbulence, and
other atmospheric effects, such as ionosphere reflections, meteor
trails, and Hail spike. Clutter may also be returned from man-made
objects such as buildings and, intentionally, by radar countermeasures
such as chaff.
Some clutter may also be caused by a long radar waveguide between the
radar transceiver and the antenna. In a typical plan position
indicator (PPI) radar with a rotating antenna, this will usually be
seen as a "sun" or "sunburst" in the centre of the display as the
receiver responds to echoes from dust particles and misguided RF in
the waveguide. Adjusting the timing between when the transmitter sends
a pulse and when the receiver stage is enabled will generally reduce
the sunburst without affecting the accuracy of the range, since most
sunburst is caused by a diffused transmit pulse reflected before it
leaves the antenna. Clutter is considered a passive interference
source, since it only appears in response to radar signals sent by the
radar.
Clutter is detected and neutralized in several ways. Clutter tends to
appear static between radar scans; on subsequent scan echoes,
desirable targets will appear to move, and all stationary echoes can
be eliminated. Sea clutter can be reduced by using horizontal
polarization, while rain is reduced with circular polarization
(meteorological radars wish for the opposite effect, and therefore use
linear polarization to detect precipitation). Other methods attempt to
increase the signal-to-clutter ratio.
Clutter moves with the wind or is stationary. Two common strategies to
improve measure or performance in a clutter environment are:

The most effective clutter reduction technique is pulse-Doppler radar.
Doppler separates clutter from aircraft and spacecraft using a
frequency spectrum, so individual signals can be separated from
multiple reflectors located in the same volume using velocity
differences. This requires a coherent transmitter. Another technique
uses a moving target indicator that subtracts the receive signal from
two successive pulses using phase to reduce signals from slow moving
objects. This can be adapted for systems that lack a coherent
transmitter, such as time-domain pulse-amplitude radar.
Constant false alarm rate, a form of automatic gain control (AGC), is
a method that relies on clutter returns far outnumbering echoes from
targets of interest. The receiver's gain is automatically adjusted to
maintain a constant level of overall visible clutter. While this does
not help detect targets masked by stronger surrounding clutter, it
does help to distinguish strong target sources. In the past, radar AGC
was electronically controlled and affected the gain of the entire
radar receiver. As radars evolved, AGC became computer-software
controlled and affected the gain with greater granularity in specific
detection cells.

Clutter may also originate from multipath echoes from valid targets
caused by ground reflection, atmospheric ducting or ionospheric
reflection/refraction (e.g., anomalous propagation). This clutter type
is especially bothersome since it appears to move and behave like
other normal (point) targets of interest. In a typical scenario, an
aircraft echo is reflected from the ground below, appearing to the
receiver as an identical target below the correct one. The radar may
try to unify the targets, reporting the target at an incorrect height,
or eliminating it on the basis of jitter or a physical impossibility.
Terrain bounce jamming exploits this response by amplifying the radar
signal and directing it downward.[36] These problems can be overcome
by incorporating a ground map of the radar's surroundings and
eliminating all echoes which appear to originate below ground or above
a certain height. Monopulse can be improved by altering the elevation
algorithm used at low elevation. In newer air traffic control radar
equipment, algorithms are used to identify the false targets by
comparing the current pulse returns to those adjacent, as well as
calculating return improbabilities.
Jamming[edit]
Main article:
RadarRadar jamming and deception
RadarRadar jamming refers to radio frequency signals originating from
sources outside the radar, transmitting in the radar's frequency and
thereby masking targets of interest. Jamming may be intentional, as
with an electronic warfare tactic, or unintentional, as with friendly
forces operating equipment that transmits using the same frequency
range. Jamming is considered an active interference source, since it
is initiated by elements outside the radar and in general unrelated to
the radar signals.
Jamming is problematic to radar since the jamming signal only needs to
travel one way (from the jammer to the radar receiver) whereas the
radar echoes travel two ways (radar-target-radar) and are therefore
significantly reduced in power by the time they return to the radar
receiver. Jammers therefore can be much less powerful than their
jammed radars and still effectively mask targets along the line of
sight from the jammer to the radar (mainlobe jamming). Jammers have an
added effect of affecting radars along other lines of sight through
the radar receiver's sidelobes (sidelobe jamming).
Mainlobe jamming can generally only be reduced by narrowing the
mainlobe solid angle and cannot fully be eliminated when directly
facing a jammer which uses the same frequency and polarization as the
radar.
SidelobeSidelobe jamming can be overcome by reducing receiving
sidelobes in the radar antenna design and by using an omnidirectional
antenna to detect and disregard non-mainlobe signals. Other
anti-jamming techniques are frequency hopping and polarization.
RadarRadar signal processing[edit]
Distance measurement[edit]
Transit time[edit]
Further information: Time of flight

Pulse radar: The round-trip time for the radar pulse to get to the
target and return is measured. The distance is proportional to this
time.

Continuous wave (CW) radar

One way to obtain a distance measurement is based on the
time-of-flight: transmit a short pulse of radio signal
(electromagnetic radiation) and measure the time it takes for the
reflection to return. The distance is one-half the product of the
round trip time (because the signal has to travel to the target and
then back to the receiver) and the speed of the signal. Since radio
waves travel at the speed of light, accurate distance measurement
requires high-speed electronics. In most cases, the receiver does not
detect the return while the signal is being transmitted. Through the
use of a duplexer, the radar switches between transmitting and
receiving at a predetermined rate. A similar effect imposes a maximum
range as well. In order to maximize range, longer times between pulses
should be used, referred to as a pulse repetition time, or its
reciprocal, pulse repetition frequency.
These two effects tend to be at odds with each other, and it is not
easy to combine both good short range and good long range in a single
radar. This is because the short pulses needed for a good minimum
range broadcast have less total energy, making the returns much
smaller and the target harder to detect. This could be offset by using
more pulses, but this would shorten the maximum range. So each radar
uses a particular type of signal. Long-range radars tend to use long
pulses with long delays between them, and short range radars use
smaller pulses with less time between them. As electronics have
improved many radars now can change their pulse repetition frequency,
thereby changing their range. The newest radars fire two pulses during
one cell, one for short range (about 10 km (6.2 mi)) and a
separate signal for longer ranges (about 100 km (62 mi)).
The distance resolution and the characteristics of the received signal
as compared to noise depends on the shape of the pulse. The pulse is
often modulated to achieve better performance using a technique known
as pulse compression.
Distance may also be measured as a function of time. The radar mile is
the time it takes for a radar pulse to travel one nautical mile,
reflect off a target, and return to the radar antenna. Since a
nautical mile is defined as 1,852 m, then dividing this distance
by the speed of light (299,792,458 m/s), and then multiplying the
result by 2 yields a result of 12.36 μs in duration.
FrequencyFrequency modulation[edit]
Main article:
FrequencyFrequency modulation
Another form of distance measuring radar is based on frequency
modulation.
FrequencyFrequency comparison between two signals is considerably
more accurate, even with older electronics, than timing the signal. By
measuring the frequency of the returned signal and comparing that with
the original, the difference can be easily measured.
This technique can be used in continuous wave radar and is often found
in aircraft radar altimeters. In these systems a "carrier" radar
signal is frequency modulated in a predictable way, typically varying
up and down with a sine wave or sawtooth pattern at audio frequencies.
The signal is then sent out from one antenna and received on another,
typically located on the bottom of the aircraft, and the signal can be
continuously compared using a simple beat frequency modulator that
produces an audio frequency tone from the returned signal and a
portion of the transmitted signal.
Since the signal frequency is changing, by the time the signal returns
to the aircraft the transmit frequency has changed. The frequency
shift is used to measure distance.
The modulation index riding on the receive signal is proportional to
the time delay between the radar and the reflector. The frequency
shift becomes greater with greater time delay. The frequency shift is
directly proportional to the distance travelled. That distance can be
displayed on an instrument, and it may also be available via the
transponder. This signal processing is similar to that used in speed
detecting Doppler radar. Example systems using this approach are
AZUSA, MISTRAM, and UDOP.
A further advantage is that the radar can operate effectively at
relatively low frequencies. This was important in the early
development of this type when high frequency signal generation was
difficult or expensive.
Terrestrial radar uses low-power FM signals that cover a larger
frequency range. The multiple reflections are analyzed mathematically
for pattern changes with multiple passes creating a computerized
synthetic image. Doppler effects are used which allows slow moving
objects to be detected as well as largely eliminating "noise" from the
surfaces of bodies of water.
SpeedSpeed measurement[edit]
SpeedSpeed is the change in distance to an object with respect to time.
Thus the existing system for measuring distance, combined with a
memory capacity to see where the target last was, is enough to measure
speed. At one time the memory consisted of a user making grease pencil
marks on the radar screen and then calculating the speed using a slide
rule. Modern radar systems perform the equivalent operation faster and
more accurately using computers.
If the transmitter's output is coherent (phase synchronized), there is
another effect that can be used to make almost instant speed
measurements (no memory is required), known as the Doppler effect.
Most modern radar systems use this principle into
Doppler radarDoppler radar and
pulse-
Doppler radarDoppler radar systems (weather radar, military radar). The
Doppler effectDoppler effect is only able to determine the relative speed of the
target along the line of sight from the radar to the target. Any
component of target velocity perpendicular to the line of sight cannot
be determined by using the
Doppler effectDoppler effect alone, but it can be
determined by tracking the target's azimuth over time.
It is possible to make a
Doppler radarDoppler radar without any pulsing, known as a
continuous-wave radar (CW radar), by sending out a very pure signal of
a known frequency. CW radar is ideal for determining the radial
component of a target's velocity. CW radar is typically used by
traffic enforcement to measure vehicle speed quickly and accurately
where range is not important.
When using a pulsed radar, the variation between the phase of
successive returns gives the distance the target has moved between
pulses, and thus its speed can be calculated. Other mathematical
developments in radar signal processing include time-frequency
analysis (Weyl Heisenberg or wavelet), as well as the chirplet
transform which makes use of the change of frequency of returns from
moving targets ("chirp").
Pulse-DopplerPulse-Doppler signal processing[edit]
Main article:
Pulse-DopplerPulse-Doppler signal processing

Pulse-DopplerPulse-Doppler signal processing. The Range Sample axis represents
individual samples taken in between each transmit pulse. The Range
Interval axis represents each successive transmit pulse interval
during which samples are taken. The Fast Fourier Transform process
converts time-domain samples into frequency domain spectra. This is
sometimes called the bed of nails.

Pulse-Doppler signal processingPulse-Doppler signal processing includes frequency filtering in the
detection process. The space between each transmit pulse is divided
into range cells or range gates. Each cell is filtered independently
much like the process used by a spectrum analyzer to produce the
display showing different frequencies. Each different distance
produces a different spectrum. These spectra are used to perform the
detection process. This is required to achieve acceptable performance
in hostile environments involving weather, terrain, and electronic
countermeasures.
The primary purpose is to measure both the amplitude and frequency of
the aggregate reflected signal from multiple distances. This is used
with weather radar to measure radial wind velocity and precipitation
rate in each different volume of air. This is linked with computing
systems to produce a real-time electronic weather map.
AircraftAircraft safety
depends upon continuous access to accurate weather radar information
that is used to prevent injuries and accidents.
Weather radarWeather radar uses a
low PRF. Coherency requirements are not as strict as those for
military systems because individual signals ordinarily do not need to
be separated. Less sophisticated filtering is required, and range
ambiguity processing is not normally needed with weather radar in
comparison with military radar intended to track air vehicles.
The alternate purpose is "look-down/shoot-down" capability required to
improve military air combat survivability.
Pulse-DopplerPulse-Doppler is also used
for ground based surveillance radar required to defend personnel and
vehicles.[37][38]
Pulse-Doppler signal processingPulse-Doppler signal processing increases the
maximum detection distance using less radiation in close proximity to
aircraft pilots, shipboard personnel, infantry, and artillery.
Reflections from terrain, water, and weather produce signals much
larger than aircraft and missiles, which allows fast moving vehicles
to hide using nap-of-the-earth flying techniques and stealth
technology to avoid detection until an attack vehicle is too close to
destroy.
Pulse-Doppler signal processingPulse-Doppler signal processing incorporates more
sophisticated electronic filtering that safely eliminates this kind of
weakness. This requires the use of medium pulse-repetition frequency
with phase coherent hardware that has a large dynamic range. Military
applications require medium PRF which prevents range from being
determined directly, and range ambiguity resolution processing is
required to identify the true range of all reflected signals. Radial
movement is usually linked with Doppler frequency to produce a lock
signal that cannot be produced by radar jamming signals. Pulse-Doppler
signal processing also produces audible signals that can be used for
threat identification.[37]
Reduction of interference effects[edit]
Signal processingSignal processing is employed in radar systems to reduce the radar
interference effects.
Signal processingSignal processing techniques include moving
target indication,
Pulse-DopplerPulse-Doppler signal processing, moving target
detection processors, correlation with secondary surveillance radar
targets, space-time adaptive processing, and track-before-detect.
Constant false alarm rateConstant false alarm rate and digital terrain model processing are
also used in clutter environments.
Plot and track extraction[edit]
Main article: Track algorithm
A
Track algorithm is a radar performance enhancement strategy.
Tracking algorithms provide the ability to predict future position of
multiple moving objects based on the history of the individual
positions being reported by sensor systems.
Historical information is accumulated and used to predict future
position for use with air traffic control, threat estimation, combat
system doctrine, gun aiming, and missile guidance. Position data is
accumulated radar sensors over the span of a few minutes.
There are four common track algorithms.[39]

RadarRadar video returns from aircraft can be subjected to a plot
extraction process whereby spurious and interfering signals are
discarded. A sequence of target returns can be monitored through a
device known as a plot extractor.
The non-relevant real time returns can be removed from the displayed
information and a single plot displayed. In some radar systems, or
alternatively in the command and control system to which the radar is
connected, a radar tracker is used to associate the sequence of plots
belonging to individual targets and estimate the targets' headings and
speeds.
Engineering[edit]
Main article:
RadarRadar engineering details

A transmitter that generates the radio signal with an oscillator such
as a klystron or a magnetron and controls its duration by a modulator.
A waveguide that links the transmitter and the antenna.
A duplexer that serves as a switch between the antenna and the
transmitter or the receiver for the signal when the antenna is used in
both situations.
A receiver. Knowing the shape of the desired received signal (a
pulse), an optimal receiver can be designed using a matched filter.
A display processor to produce signals for human readable output
devices.
An electronic section that controls all those devices and the antenna
to perform the radar scan ordered by software.
A link to end user devices and displays.

Antenna design[edit]
Main article: Antenna (radio)
RadioRadio signals broadcast from a single antenna will spread out in all
directions, and likewise a single antenna will receive signals equally
from all directions. This leaves the radar with the problem of
deciding where the target object is located.
Early systems tended to use omnidirectional broadcast antennas, with
directional receiver antennas which were pointed in various
directions. For instance, the first system to be deployed, Chain Home,
used two straight antennas at right angles for reception, each on a
different display. The maximum return would be detected with an
antenna at right angles to the target, and a minimum with the antenna
pointed directly at it (end on). The operator could determine the
direction to a target by rotating the antenna so one display showed a
maximum while the other showed a minimum. One serious limitation with
this type of solution is that the broadcast is sent out in all
directions, so the amount of energy in the direction being examined is
a small part of that transmitted. To get a reasonable amount of power
on the "target", the transmitting aerial should also be directional.
Parabolic reflector[edit]
Main article: Parabolic antenna
More modern systems use a steerable parabolic "dish" to create a tight
broadcast beam, typically using the same dish as the receiver. Such
systems often combine two radar frequencies in the same antenna in
order to allow automatic steering, or radar lock.
Parabolic reflectors can be either symmetric parabolas or spoiled
parabolas: Symmetric parabolic antennas produce a narrow "pencil" beam
in both the X and Y dimensions and consequently have a higher gain.
The
NEXRADNEXRADPulse-DopplerPulse-Doppler weather radar uses a symmetric antenna to
perform detailed volumetric scans of the atmosphere. Spoiled parabolic
antennas produce a narrow beam in one dimension and a relatively wide
beam in the other. This feature is useful if target detection over a
wide range of angles is more important than target location in three
dimensions. Most 2D surveillance radars use a spoiled parabolic
antenna with a narrow azimuthal beamwidth and wide vertical beamwidth.
This beam configuration allows the radar operator to detect an
aircraft at a specific azimuth but at an indeterminate height.
Conversely, so-called "nodder" height finding radars use a dish with a
narrow vertical beamwidth and wide azimuthal beamwidth to detect an
aircraft at a specific height but with low azimuthal precision.

Primary Scan: A scanning technique where the main antenna aerial is
moved to produce a scanning beam, examples include circular scan,
sector scan, etc.
Secondary Scan: A scanning technique where the antenna feed is moved
to produce a scanning beam, examples include conical scan,
unidirectional sector scan, lobe switching, etc.
Palmer Scan: A scanning technique that produces a scanning beam by
moving the main antenna and its feed. A Palmer Scan is a combination
of a Primary Scan and a Secondary Scan.
Conical scanning: The radar beam is rotated in a small circle around
the "boresight" axis, which is pointed at the target.

Main article: Slotted waveguide
Applied similarly to the parabolic reflector, the slotted waveguide is
moved mechanically to scan and is particularly suitable for
non-tracking surface scan systems, where the vertical pattern may
remain constant. Owing to its lower cost and less wind exposure,
shipboard, airport surface, and harbour surveillance radars now use
this approach in preference to a parabolic antenna.
Phased array[edit]

Phased array: Not all radar antennas must rotate to scan the sky.

Main article: Phased array
Another method of steering is used in a phased array radar.
Phased arrayPhased array antennas are composed of evenly spaced similar antenna
elements, such as aerials or rows of slotted waveguide. Each antenna
element or group of antenna elements incorporates a discrete phase
shift that produces a phase gradient across the array. For example,
array elements producing a 5 degree phase shift for each wavelength
across the array face will produce a beam pointed 5 degrees away from
the centreline perpendicular to the array face. Signals travelling
along that beam will be reinforced. Signals offset from that beam will
be cancelled. The amount of reinforcement is antenna gain. The amount
of cancellation is side-lobe suppression.[40]
Phased arrayPhased array radars have been in use since the earliest years of radar
in
World War IIWorld War II (Mammut radar), but electronic device limitations led
to poor performance.
Phased arrayPhased array radars were originally used for
missile defence (see for example Safeguard Program). They are the
heart of the ship-borne
Aegis Combat SystemAegis Combat System and the Patriot Missile
System. The massive redundancy associated with having a large number
of array elements increases reliability at the expense of gradual
performance degradation that occurs as individual phase elements fail.
To a lesser extent,
Phased arrayPhased array radars have been used in Weather
Surveillance. As of 2017, NOAA plans to implement a national network
of Multi-Function
Phased arrayPhased array radars throughout the United States
within 10 years, for meteorological studies and flight monitoring.[41]
Phased arrayPhased array antenna can be built to conform to specific shapes, like
missiles, infantry support vehicles, ships, and aircraft.
As the price of electronics has fallen, phased array radars have
become more common. Almost all modern military radar systems are based
on phased arrays, where the small additional cost is offset by the
improved reliability of a system with no moving parts. Traditional
moving-antenna designs are still widely used in roles where cost is a
significant factor such as air traffic surveillance and similar
systems.
Phased arrayPhased array radars are valued for use in aircraft since they can
track multiple targets. The first aircraft to use a phased array radar
was the B-1B Lancer. The first fighter aircraft to use phased array
radar was the Mikoyan MiG-31. The MiG-31M's SBI-16
ZaslonZaslon Passive
electronically scanned array radar was considered to be the world's
most powerful fighter radar,[42] until the
AN/APG-77AN/APG-77 Active
electronically scanned array was introduced on the Lockheed Martin
F-22 Raptor.
Phased-array interferometry or aperture synthesis techniques, using an
array of separate dishes that are phased into a single effective
aperture, are not typical for radar applications, although they are
widely used in radio astronomy. Because of the thinned array curse,
such multiple aperture arrays, when used in transmitters, result in
narrow beams at the expense of reducing the total power transmitted to
the target. In principle, such techniques could increase spatial
resolution, but the lower power means that this is generally not
effective.
Aperture synthesis by post-processing motion data from a single moving
source, on the other hand, is widely used in space and airborne radar
systems.
FrequencyFrequency bands[edit]
Main article:
Radio spectrumRadio spectrum § IEEE
The traditional band names originated as code-names during World War
II and are still in military and aviation use throughout the world.
They have been adopted in the
United StatesUnited States by the Institute of
Electrical and Electronics Engineers and internationally by the
International Telecommunication Union. Most countries have additional
regulations to control which parts of each band are available for
civilian or military use.
Other users of the radio spectrum, such as the broadcasting and
electronic countermeasures industries, have replaced the traditional
military designations with their own systems.

X
8–12 GHz
2.5–3.75 cm
MissileMissile guidance, marine radar, weather, medium-resolution mapping and
ground surveillance; in the
United StatesUnited States the narrow range
10.525 GHz ±25 MHz is used for airport radar; short range
tracking. Named
X bandX band because the frequency was a secret during WW2.

K
18–24 GHz
1.11–1.67 cm
From German kurz, meaning 'short'; limited use due to absorption by
water vapour, so Ku and Ka were used instead for surveillance. K-band
is used for detecting clouds by meteorologists, and by police for
detecting speeding motorists. K-band radar guns operate at 24.150 ±
0.100 GHz.

mm
40–300 GHz
1.0–7.5 mm
MillimetreMillimetre band, subdivided as below. The frequency ranges depend on
waveguide size. Multiple letters are assigned to these bands by
different groups. These are from Baytron, a now defunct company that
made test equipment.

RadarRadar modulators[edit]
Modulators act to provide the waveform of the RF-pulse. There are two
different radar modulator designs:

High voltage switch for non-coherent keyed power-oscillators[43] These
modulators consist of a high voltage pulse generator formed from a
high voltage supply, a pulse forming network, and a high voltage
switch such as a thyratron. They generate short pulses of power to
feed, e.g., the magnetron, a special type of vacuum tube that converts
DC (usually pulsed) into microwaves. This technology is known as
pulsed power. In this way, the transmitted pulse of RF radiation is
kept to a defined and usually very short duration.
Hybrid mixers,[44] fed by a waveform generator and an exciter for a
complex but coherent waveform. This waveform can be generated by low
power/low-voltage input signals. In this case the radar transmitter
must be a power-amplifier, e.g., a klystron tube or a solid state
transmitter. In this way, the transmitted pulse is
intrapulse-modulated and the radar receiver must use pulse compression
techniques.

RadarRadar coolant[edit]
Main article: Coolant
Coherent microwave amplifiers operating above 1,000 watts microwave
output, like travelling wave tubes and klystrons, require liquid
coolant. The electron beam must contain 5 to 10 times more power than
the microwave output, which can produce enough heat to generate
plasma. This plasma flows from the collector toward the cathode. The
same magnetic focusing that guides the electron beam forces the plasma
into the path of the electron beam but flowing in the opposite
direction. This introduces FM modulation which degrades Doppler
performance. To prevent this, liquid coolant with minimum pressure and
flow rate is required, and deionized water is normally used in most
high power surface radar systems that utilize Doppler processing.[45]
Coolanol (silicate ester) was used in several military radars in the
1970s. However, it is hygroscopic, leading to hydrolysis and formation
of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978
was attributed to a silicate ester fire.[46]
Coolanol is also
expensive and toxic. The U.S. Navy has instituted a program named
Pollution Prevention (P2) to eliminate or reduce the volume and
toxicity of waste, air emissions, and effluent discharges. Because of
this,
Coolanol is used less often today.
Regulations[edit]
RadarRadar (also: RADAR) is defined by article 1.100 of the International
Telecommunication Union´s (ITU)
ITU Radio Regulations (RR) as:[47]

A radiodetermination system based on the comparison of reference
signals with radio signals reflected, or retransmitted, from the
position to be determined. Each radiodetermination system shall be
classified by the radiocommunication service in which it operates
permanently or temporarily. Typical radar utilizations are primary
radar and secondary radar, these might operate in the radiolocation
service or the radiolocation-satellite service.

See also[edit]

Electronics portal
Nautical portal

Main article:
RadarRadar configurations and types
Main category: Radar

This "further reading" section may contain inappropriate and/or
excessive suggestions. Please ensure that only a reasonable number of
balanced, topical, reliable, and notable further reading suggestions
are given. Consider utilising appropriate texts as inline sources or
creating a separate bibliography article. (November 2014)

References[edit]

Barrett, Dick, "All you ever wanted to know about British air defence
radar". The
RadarRadar Pages. (History and details of various British radar
systems)
Buderi, "Telephone History:
RadarRadar History". Privateline.com.
(Anecdotal account of the carriage of the world's first high power
cavity magnetron from Britain to the US during WW2.)
Ekco
RadarRadar WW2 Shadow Factory The secret development of British radar.
ES310 "Introduction to Naval Weapons Engineering.". (Radar
fundamentals section)
Hollmann, Martin, "
RadarRadar Family Tree".
RadarRadar World.
Penley, Bill, and Jonathan Penley, "Early
RadarRadar History—an
Introduction". 2002.
Pub 1310
RadarRadar Navigation and Maneuvering Board Manual, National
Imagery and Mapping Agency, Bethesda, MD 2001 (US govt publication
'...intended to be used primarily as a manual of instruction in
navigation schools and by naval and merchant marine personnel.')
Wesley Stout, 1946 "
RadarRadar - The Great Detective" Early development and
production by Chrysler Corp. during WWII.
Swords, Seán S., "Technical History of the Beginnings of Radar", IEE
History of Technology Series, Vol. 6, London: Peter Peregrinus, 1986